In general, with an anti-lock control device for a vehicle, anti-lock control is effected by means of a microcomputer such that hold valves and decay valves are opened and closed on the basis of electrical signals representing wheel speeds detected by wheel speed sensors, thereby increasing, holding or decreasing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability, while at the same time reducing the braking distance of the vehicle.
FIG. 1 shows control state diagrams as disclosed in U.S. Pat. No. 4,741,580 which illustrate the changes in the wheel speed Vw, the wheel acceleration and deceleration Vw and the brake hydraulic pressure Pw, as well as a hold signal HS and a decay signal DS for opening and closing the hold valves and the decay valves.
In a state of the vehicle in running where no brake is operated, the brake hydraulic pressure Pw is not increased and both of the hold signal HS and the decay signal DS are in the off-state, so that the hold valve is in the open state whereas the decay valve is in the closed state. However, with a brake operation, the brake hydraulic pressure Pw increases rapidly from time point t0 (normal mode), reducing the wheel speed Vw. There is set up a reference wheel speed Vr which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. The reference wheel speed Vr is set up so that when the wheel deceleration (negative acceleration) Vw of the wheel attains a predetermined threshold value, -1G, for instance, at a time point t1, it decreases linearly in time from the time point t1 with a slope .theta. for the deceleration of -1G.
At a time point t2 when the wheel deceleration Vw reaches a predetermined value -Gmax with maximum absolute value, the hold valve closes by turning on the hold signal HS to hold the brake hydraulic pressure Pw.
With the holding of the brake hydraulic pressure Pw in such a manner, the wheel speed Vw further decreases to become less than the reference wheel speed Vr beyond a time point t3. At that time point t3 the decay signals DS is turned on to open the decay valve to start reducing the brake hydraulic pressure Pw. As a result of the pressure reduction, the wheel speed Vw is shifted from decrease to increase at a time point t4 when a low peak Vl of the wheel speed Vw occurs. At the time point t4 of the low peak, the decay signal DS is turned off to close the decay valve, so that the reduction of the brake hydraulic pressure Pw is completed and the brake hydraulic pressure Pw is held at the value at that time.
Next, when the wheel speed Vw attains a high peak Vh at a time point t7, an increase in the brake hydraulic pressure takes place again. The pressure increase in the brake hydraulic pressure Pw and the decrease in the wheel speed Vw in this stage is arranged to take place gradually by a repetition of turning on and off of the hold signal HS mincingly. Starting at a time point t8 (corresponding to t3) a decompression mode is generated again.
It is to be noted that during the above operation, a time point t5 at which the wheel speed Vw is recovered to a speed Vb (=Vl+0.15Y) where Vl is the wheel speed at the low peak and Y is the difference between the wheel speed Va at the time point t3 and the low peak speed Vl, so that Vb represents the wheel speed at which 15% of the speed difference Y is gained from the low peak value Vl, and a time point t6 at which the wheel speed increases to Vc (=Vl+0.8Y) where 80% of the speed difference Y is gained from the low peak speed Vl, are detected. Further, the interval Tx of the first pressurization which starts at the time point t7 is determined by the judgment on the friction coefficient .mu. of the road surface as obtained based on the computation of the average acceleration (Vc-Vb) / .DELTA.T for the period .DELTA.T between the time point t5 and t6. In addition, the holding periods or the pressurization periods that follow are determined based on the vehicle deceleration Vw that are detected immediately before each holding or pressurization. Through a combination of augmentation, holding and reduction of the brake hydraulic pressure Pw as described in the above, it is possible to reduce the vehicle speed by controlling the wheel speed Vw without causing the locking of the wheels.
Now, as is clear from FIG. 1, in the conventional anti-lock control method, the brake hydraulic pressure Pw increases again at the time point t.sub.7 where the wheel speed Vw that has once been decelerated due to an increase in the brake hydraulic pressure Pw recovers the speed by the subsequent decrease and holding of the brake hydraulic pressure Pw and attains the high peak. However, when the pressure increase is started from the time point t.sub.7 of the high peak, the pressure increase tends to lag when the vehicle is on a road with high value of the friction coefficient .mu., thereby causing a drawback that the braking distance is increased. At the same time, on a road with low value of the friction coefficient .mu., a pressure increase starts even when the wheel speed Vw is shifted temporarily from acceleration to deceleration before the wheel speed Vw is fully recovered. Therefore, there was a drawback in that the wheel speed Vw drops further deep to generate a locking of the wheels.
In view of the foregoing drawbacks, there has been proposed to start the pressure increase, not at the high peak point of the wheel speed Vw but rather at the time point where the wheel speed Vw attains (Vv-.DELTA.V) which is the speed lower by a predetermined value .DELTA.V than the computed vehicle speed Vv. However, when the starting point of pressure increase is advanced in this manner, there still has a problem that the starting point of the pressure increase is too premature, giving rise to an excessively pressure increased state, if, for example, the road surface condition is changed suddenly from that of a high friction coefficient .mu. value to that of a low .mu. value.
SUMMARY OF THE INVENTION
It is, therefore, the object of the present invention to provide an anti-lock control method for a vehicle which can deal with a sudden change in the friction coefficient .mu. of the road surface.
The foregoing object can be achieved by a provision of an anti-lock method and system in which, according to the present invention, the pressure increase of the brake hydraulic pressure Pw starts again at a time point where the wheel speed Vw, which has been decelerated due to an increase in the brake hydraulic pressure Pw, recovers the speed due to the subsequent pressure decrease and holding of the brake hydraulic pressure Pw and reaches a speed (Vv-.DELTA.Vx) which is lower than the computed vehicle speed Vv by a predetermined value .DELTA.Vx, where the predetermined value .DELTA.Vx varies in accordance with the magnitude of the friction coefficient .mu. of the road surface.
The value of Vx is arranged to vary to be small when the friction coefficient .mu. of the road surface is low, and large when the friction coefficient .mu. of the road surface is high.